In T192510#2827759, @Caius-Corsades wrote:

Yes, I think it's something like that. It happened to me 5 or 6 times on the official server. I was in a car, entering a clear town, and then suddenly the mist appeared. I think that if you don't receive the data for the event starting, you don't see the mist — just like when you're too far during a bombing, or after disconnecting and reconnecting.

Same here. It’s been a few days we have the same problem on official vanilla. We ran into two gas zones with no visual gas. Just happened to me in Novy today, and last time was on the military zone on south Dubrovka. I recorded the event if necessary. Both times, it was a dynamic gas event we didn’t hear spawn, no gas bomb sound. When we hear the sound, the gas is always visible. Last one in Novy was interesting cause I heard the event, and I saw the gas first. I chose to wait for the gas to disappear and logged off my server. When I logged back 20 minutes later, no gas was visible anymore. I thought is was gone so I entered Novy and got sick immediately - playing on Xbox